1. Field of the Invention
The present invention relates to an improvement in joints intended in particular for the connection of conductors of corresponding ends in single or multi-core electric cables for medium and high voltages, insulated with an extruded dielectric material, for example, saturated chain ethylene-propylene copolymer or ethylene-propylene-diene terpolymer, polyethylene, crosslinked polyethylene, etc.
2. Brief Description of the Prior Art
In the field of conductors for the transmission of high-currents for power distribution purposes, it is often necessary to provide suitable electrical and mechanical connection between two or more high-current conductors, for example between such conductors of the same or different sizes or between such a conductor and a terminal. Furthermore, particularly in field operations such as the outdoor laying or repairing of underground cables, it becomes important to be able to provide such junctions quickly, without elaborate equipment and without the use of equipment which requires specialized skills or critical or dangerous operations, or which is unsuited to outdoor use. In addition, it is generally desirable that such junctions be as short as possible, not only from considerations of expense of material and space required, but also because the conductor normally must be electrically insulated and the junction must therefore be thoroughly covered with insulating material such as electrical tape or other suitable insulating material. The time and materials required for the necessary thorough taping or other insulating procedure becomes greater the greater the length of the junction region.
Corona problems develop whenever sufficient electrical voltage difference is present between spaced conductors, separated by air spacing or air pockets. Corona, in addition to wasting power, is deleterious to many insulating materials and often causes physical deterioration and rapid chemical decomposition thereof. In addition, corona envelopes cause a radio interference and noise which is objectionable.
It is also known to join high-current conductors by butt-welding of their adjacent ends. While such welding tends to permit use of shorter junctions for the same maximum current, welding often involves so many problems and safety hazards that such procedures are usually considered very undesirable, particularly in field service or installation applications. For example, in a typical power-distribution system so many primary and secondary splices of large-diameter conductors need to be made that the wide-spread use of welding equipment entails not only costly investment in the equipment, but also troublesome problems with scheduling the use of the equipment; additionally, skilled welders are generally in short supply. Importantly also, the use of welding equipment creates substantial problems of fire and explosion hazard.
One of the methods at present in use for constructing joints between conductors with extruded insulation consists in shaping the end of the insulation similar to a pencil point, and in rebuilding it, after the connection of the conductors, by means of a winding of plastic or rubber tape of dielectric material appropriately wound up in a cigar-wrapper-like fashion. As is known, said process is slow and requires highly specialized personnel and is, therefore, expensive.
Further, despite the skill of the operators, there is always the possibility of formation of wrinkles in the wrapping, in which air can be trapped, with the consequent effect of disruptive discharges when the cable is in use.
Another conventional method is that known in this field by the name "pennant" type lapping. In this case, the insulation in the jointing zone is restored with the aid of a previously shaped foil of rubber or plastic material. However, in this case, also, there is the risk of stretching the foil too much or too little, during its application, with the possible formation of wrinkles or pockets in which air might be retained.
Another method provides the rebuilding of the joints by means of special tapes between which is subsequently injected a resin capable of solidifying by polymerization. In this cases as well, the method is slow, specialized labor is necessary, and there is still the risk the air bubbles may be trapped in the wrinkles of the winding and may not be reached by the resin. Also, there is the danger that air bubbles will be entrapped in the resin.
Another method consists in applying half-shells made of polyethylene around the joining zone. Said half-shells have the object of forming means for receiving a resin which solidifies by polymerization. The variations in the diameter of the cable insulation during the cable use, due, for example, to the thermal cycles, can easily cause the detachment of the insulation from the resin, with the consequent formation of zones in which air is present.
According to another method, the insulation is rebuilt by applying a pre-shaped sleeve of vulcanized rubber, having an inner diameter smaller than the diameter of the cable core. The sleeve is forced on the core, and it is, therefore, necessary, for each core having a certain nominal diameter, to have available a plurality of sleeves having a different diameter in order to take into accounts the processing tolerances.
According to another method, the joint is carried out by means of sleeves provided with a longitudinal cavity or bore of such a diameter as to be able to slide on the cable core, and the space between the inner surface of said longitudinal cavity and, respectively, the core and the conductor is filled with small cylinders or adaptors, made of layers of material. It is known that the greater number of layers or elements of a joint, the greater the risk of air trapping. Moreover, the work of installation requires substantial time, and the cost of the joint is increased.